Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192140
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
340 PM CST Fri Jan 19 2018

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 339 PM CST Fri Jan 19 2018

Clockwise flow around the surface high currently over the
southeastern CONUS combined with a developing low over the western
Plains is producing south-southwest flow over the Mid Mississippi
Valley today.  The southerly flow will continue tonight and Saturday
keeping mild temperatures over the area.  Temperatures have already
hit the low to mid 50s across much of the area as of 300 PM, and
similar if not slightly warmer temperatures are likely on Saturday.
The one potential problem for getting much warmer than today is
clouds.  Current satellite pics show an expansive area of clouds
over eastern Texas, Louisiana, and Arkansas.  Short range guidance
is pretty insistent that this area of clouds will expand northward
tonight to spread into the Mid Mississippi Valley.  Latest thinking
is that the clouds will reach central and southeast Missouri an hour
or two before sunrise and continue spreading north or northeast
through the morning.  The clouds will likely put a bit of a damper
on Saturday`s warmup, but warm advection will continue through the
day so think there`s a good chance temperatures will warm back up
well into the 50s.

Carney

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 339 PM CST Fri Jan 19 2018

A strong shortwave trof will move from the West Coast across the
Rockies on Saturday night.  The trof will move into the western
Plains on Sunday and pick the surface low up...forcing cyclogenesis
and pushing it northeast from the Oklahoma Panhandle Sunday morning
into Iowa by Monday.  The low will push a strong cold front through
Missouri and Illinois Sunday night and Monday morning.  Temperatures
will fall from the mid 50s to around 60 on Sunday to the 40s behind
the front on Monday and down to the upper 30s to mid 40s on Monday.
There should be plenty of low level moisture over the Mid
Mississippi Valley ahead of the front to produce precipitation...so
guidance PoPs in the likely to categorical range Sunday night and
Monday morning as the front sweeps through look very reasonable.
Thunderstorms slight chance of thunderstorms still looks reasonable
as MUCAPE remains in the 200-400 J/Kg range Sunday night.  There`s a
ton of deep layer shear in the atmosphere ahead of the front as one
would expect with a cool season storm system...but the low MUCAPE
values should keep the threat for severe weather at a minimal level.
 That being said, the storm system bears watching because it would
take a whole lot more instability to really increase the severe
chances significantly.

The pattern aloft remains quasi-zonal in the wake of the Sunday
night/Monday storm system.  The surface system will lift northeast
into the Canadian Maritimes by Thursday and ridging aloft will build
over the central U.S.  This pattern will keep the coldest air behind
the system bottled up over the Upper Midwest and southern Canada.
While temperatures will be colder behind the system on Monday and
Tuesday...this won`t be an Arctic deep freeze like we`ve seen behind
cold fronts in the last few weeks.  Current medium range guidance
puts lows in the 20s Tuesday and Wednesday morning with highs
warming into the 40s for most locations.  Warm advection ramps up
again ahead of the next downstream trof on Thursday and continues
Friday.  Lows Thursday morning look likely to drop to just below
freezing in the upper 20s and low 30s with highs rebounding into the
50s both Thursday and Friday.

Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1154 AM CST Fri Jan 19 2018

VFR flight conditions will prevail across the forecast area
through this afternoon and for most of tonight as well. Southwest
flow gusting to around 25kts will cause crosswind issues on
northwest-southeast oriented runways this afternoon...but gusts
should diminish this evening. Low level wind shear is a
possibility tonight...although wind speed aloft is marginal for
including in the TAF a this time. The next potential issue will be
an area of stratus that guidance is forecasting to move up across
Missouri and into Illinois between 10-12Z Saturday. Some of the
guidance is quite low with this stratus...in the 500-1000 ft
range, while other models keep the area stratus free. Think a
middle of the road approach in MVFR is most likely at this time.
Once the stratus moves in, it`s likely to stay MVFR through
Saturday.

SPECIFICS FOR KSTL:

VFR flight conditions with gusty southwest flow is expected to
prevail at Lambert through this afternoon. The wind will be pretty
close to a direct crosswind on the main runways this
afternoon...but should become more southerly this evening with
diminishing gusts. There`s some potential for low level wind shear
with west-southwest flow aloft tonight, but the the strength of
the shear is marginal for inclusion in the TAF at this time.
Expect the stratus currently over Arkansas and Texas to move
north-northeast tonight and will move over Lambert early Saturday
morning. Some guidance has IFR ceilings with this stratus, while
others have no clouds at all on Saturday. Decided to go middle of
the road with MVFR at this time with the ceilings sticking around
through the day.

Carney

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     37  56  43  60 /   0   5  20  20
Quincy          34  49  39  54 /   0  10  30  30
Columbia        37  56  43  60 /   0  10  20  20
Jefferson City  38  58  43  63 /   0   5  20  20
Salem           35  51  41  55 /   0   5  30  20
Farmington      35  51  44  58 /   0   5  30  20

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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